Scientists refer to brain lateralization as the fact that both the halves of our brain are not exactly alike, and each hemisphere is specialized functions, and some of these functions are localized in one side or the other of the brain.
In the 1800s a French neurosurgeon named Paul Broca identified a particular area of the left hemisphere that has a major role in the production of speech. In human beings the most obvious specialized function is language and speech. Not too long after Broca a German neurologist, Carl Wernicke, was able to identify the left-hemisphere portion of the brain primarily used for language comprehension (Note: these areas are named after the scientist who discovered them).
So, when looking at a photo of a hand, how does your mind know if you are looking at your left or your right hand? This question would fall under the realm of “hand laterality” and tells a lot about how the brain “sorts out confusing perceptions.”
“For decades, the theory was that you use your motor imagination,” says Shivakumar Viswanathan, who conducted the study with colleagues Courtney Fritz and Scott T. Grafton. Psychologists used to think we simply flipped the image in our heads to ascertain which hand was pictured. Scientists at that time believed the same brain process was used to command muscles to move, which used high-level cognitive skills.
According to the scientists, our brains do not do mentally flip the images, but decodes them. Hand laterality is “a low-level sensory problem that uses processes that brings different senses into register – a process called binding,” says Viswanathan.
Participants in the study were asked to hold their hands palm down, but they were not allowed to look at them. They were able to see hand shapes shown to them at different angles. A colored dot indicated if the palm was up or down. One group saw the dot first, the other the palm shape first. Both sets of subjects mentally joined the dot and the shape, and indicated which hand was working the button.
When the image and the dot were presented simultaneously, however, those in the first group felt right hand movement when seeing a left hand, and felt a left hand movement when seeing a right hand. The second group always felt a movement of the correct hand. The perception differences were established earlier when the senses made the decision.
Participants were told which hand was correct in a second experiment, and they had to determine whether the palm was up or down. They were to answer using one hand only. “This time, the illusory hand-movement occurred only when the seen hand-shape matched that of the participant’s own palm-down responding hand, but not otherwise. Even though no right/left judgments were required, the response was dominated by an automatic binding of the seen and felt hands, and the illusory movement followed,” says Viswanathan.
This helps scientists to understand how people who have lost a limb still feel as if the limb is there, or that it itches. By placing the patient in front of a mirror the patient is able to relive the itch by looking at the side of their body that is intact. This is a “binding” of feeling and vision.
About the author:
Ron White is a two-time U.S.A. Memory Champion and memory training expert. As a memory keynote speaker he travels the world to speak before large groups or small company seminars, demonstrating his memory skills and teaching others how to improve their memory, and how important a good memory is in all phases of your life.
Science News – Right Hand or Left? How the Brain Solves a Perceptual Puzzle: http://www.sciencedaily.com/releases/2012/02/120209102007.htm
Indiana University — What is Brain Lateralization? http://www.indiana.edu/~primate/brain.html